diff --git a/openmc/data/data.py b/openmc/data/data.py
index 45da644acc..14a279b408 100644
--- a/openmc/data/data.py
+++ b/openmc/data/data.py
@@ -5,7 +5,9 @@ import re
 
 # Isotopic abundances from Meija J, Coplen T B, et al, "Isotopic compositions
 # of the elements 2013 (IUPAC Technical Report)", Pure. Appl. Chem. 88 (3),
-# pp. 293-306 (2013).
+# pp. 293-306 (2013). The "representative isotopic abundance" values from
+# column 9 are used except where an interval is given, in which case the
+# "best measurement" is used.
 NATURAL_ABUNDANCE = {
     'H1': 0.99984426, 'H2': 0.00015574, 'He3': 0.000002,
     'He4': 0.999998, 'Li6': 0.07589, 'Li7': 0.92411,
@@ -49,7 +51,7 @@ NATURAL_ABUNDANCE = {
     'Mo98': 0.24292, 'Mo100': 0.09744, 'Ru96': 0.0554,
     'Ru98': 0.0187, 'Ru99': 0.1276, 'Ru100': 0.126,
     'Ru101': 0.1706, 'Ru102': 0.3155, 'Ru104': 0.1862,
-    'Rh103': 1, 'Pd102': 0.0102, 'Pd104': 0.1114,
+    'Rh103': 1.0, 'Pd102': 0.0102, 'Pd104': 0.1114,
     'Pd105': 0.2233, 'Pd106': 0.2733, 'Pd108': 0.2646,
     'Pd110': 0.1172, 'Ag107': 0.51839, 'Ag109': 0.48161,
     'Cd106': 0.01245, 'Cd108': 0.00888, 'Cd110': 0.1247,
@@ -62,15 +64,15 @@ NATURAL_ABUNDANCE = {
     'Sb123': 0.4279, 'Te120': 0.0009, 'Te122': 0.0255,
     'Te123': 0.0089, 'Te124': 0.0474, 'Te125': 0.0707,
     'Te126': 0.1884, 'Te128': 0.3174, 'Te130': 0.3408,
-    'I127': 1, 'Xe124': 0.00095, 'Xe126': 0.00089,
+    'I127': 1.0, 'Xe124': 0.00095, 'Xe126': 0.00089,
     'Xe128': 0.0191, 'Xe129': 0.26401, 'Xe130': 0.04071,
     'Xe131': 0.21232, 'Xe132': 0.26909, 'Xe134': 0.10436,
-    'Xe136': 0.08857, 'Cs133': 1, 'Ba130': 0.0011,
+    'Xe136': 0.08857, 'Cs133': 1.0, 'Ba130': 0.0011,
     'Ba132': 0.001, 'Ba134': 0.0242, 'Ba135': 0.0659,
     'Ba136': 0.0785, 'Ba137': 0.1123, 'Ba138': 0.717,
     'La138': 0.0008881, 'La139': 0.9991119, 'Ce136': 0.00186,
     'Ce138': 0.00251, 'Ce140': 0.88449, 'Ce142': 0.11114,
-    'Pr141': 1, 'Nd142': 0.27153, 'Nd143': 0.12173,
+    'Pr141': 1.0, 'Nd142': 0.27153, 'Nd143': 0.12173,
     'Nd144': 0.23798, 'Nd145': 0.08293, 'Nd146': 0.17189,
     'Nd148': 0.05756, 'Nd150': 0.05638, 'Sm144': 0.0308,
     'Sm147': 0.15, 'Sm148': 0.1125, 'Sm149': 0.1382,
@@ -78,12 +80,12 @@ NATURAL_ABUNDANCE = {
     'Eu151': 0.4781, 'Eu153': 0.5219, 'Gd152': 0.002,
     'Gd154': 0.0218, 'Gd155': 0.148, 'Gd156': 0.2047,
     'Gd157': 0.1565, 'Gd158': 0.2484, 'Gd160': 0.2186,
-    'Tb159': 1, 'Dy156': 0.00056, 'Dy158': 0.00095,
+    'Tb159': 1.0, 'Dy156': 0.00056, 'Dy158': 0.00095,
     'Dy160': 0.02329, 'Dy161': 0.18889, 'Dy162': 0.25475,
-    'Dy163': 0.24896, 'Dy164': 0.2826, 'Ho165': 1,
+    'Dy163': 0.24896, 'Dy164': 0.2826, 'Ho165': 1.0,
     'Er162': 0.00139, 'Er164': 0.01601, 'Er166': 0.33503,
     'Er167': 0.22869, 'Er168': 0.26978, 'Er170': 0.1491,
-    'Tm169': 1, 'Yb168': 0.00123, 'Yb170': 0.02982,
+    'Tm169': 1.0, 'Yb168': 0.00123, 'Yb170': 0.02982,
     'Yb171': 0.14086, 'Yb172': 0.21686, 'Yb173': 0.16103,
     'Yb174': 0.32025, 'Yb176': 0.12995, 'Lu175': 0.97401,
     'Lu176': 0.02599, 'Hf174': 0.0016, 'Hf176': 0.0526,
@@ -96,13 +98,13 @@ NATURAL_ABUNDANCE = {
     'Os190': 0.2626, 'Os192': 0.4078, 'Ir191': 0.373,
     'Ir193': 0.627, 'Pt190': 0.00012, 'Pt192': 0.00782,
     'Pt194': 0.32864, 'Pt195': 0.33775, 'Pt196': 0.25211,
-    'Pt198': 0.07356, 'Au197': 1, 'Hg196': 0.0015,
+    'Pt198': 0.07356, 'Au197': 1.0, 'Hg196': 0.0015,
     'Hg198': 0.1004, 'Hg199': 0.1694, 'Hg200': 0.2314,
     'Hg201': 0.1317, 'Hg202': 0.2974, 'Hg204': 0.0682,
     'Tl203': 0.29524, 'Tl205': 0.70476, 'Pb204': 0.014,
     'Pb206': 0.241, 'Pb207': 0.221, 'Pb208': 0.524,
-    'Bi209': 1, 'Th230': 0.0002, 'Th232': 0.9998,
-    'Pa231': 1, 'U234': 0.000054, 'U235': 0.007204,
+    'Bi209': 1.0, 'Th230': 0.0002, 'Th232': 0.9998,
+    'Pa231': 1.0, 'U234': 0.000054, 'U235': 0.007204,
     'U238': 0.992742
 }